In the field of optical communication and optical data processing, nonlinear optical elements play an important role. A nonlinear optical material to be used for such nonlinear elements is a substance which has extremely important effects on optical signal processing, such as light mixing resulting in the generation of a frequency of sum or difference of two kinds of incident beams having different frequencies, parameteric excitation resulting in the generation of two kinds of lights having different frequencies, Pockels effect or Kerr effect causing the change in the refractive index of light media, conversion of incident beam into secondary harmonics (i.e., SHG) or tertiary harmonics (i.e., THG) and further light bistabilization. As such a nonlinear optical material there has heretofore been mainly used an inorganic material.
As such an inorganic nonlinear optical material, there has been an inorganic crystalline compound such as KDP(KH.sub.2 PO.sub.4) and lithium niobate (LiNbO.sub.3). However, such an inorganic crystalline compound leaves is still insufficient.
On the other hand, organic nonlinear optical materials have recently attracted attention as new optical element materials in the field of optoelectronics and thus have been extensively studied more and more every year. In particular, organic compounds having .pi. electron conjugated system have been extensively studied for searching of excellent materials in view of excellent properties and high responce of their simple substance.
In general, organic nonlinear optical materials in the crystalline form have a SHG coefficient of 10 to 100 times that of inorganic nonlinear optical materials and a light responce of about 1,000 times that of inorganic nonlinear optical materials. Organic nonlinear optical materials have also been known to exhibit a great shreshold with respect to damage by light.
Examples of organic nonlinear optical materials which have recently been disclosed include organic compounds such as 2-methyl-4-nitroaniline, m-nitroaniline, N-(4-nitrophenyl)-L-prolinol, 2-acetylamino-4-nitro-N,N-dimethylaniline, 4-dimethylamino-4'-nitrostilbene, 4'-dimethylamino-N-methyl-4-stilbazoriummethylsulfate, and 4'-methylbenzylidene-4-nitroaniline. The light nonlinearity of these organic compounds having .pi. electron conjugated system is attributed to the interaction between laser beams as electromagnetic wave and .pi. electron in the organic compounds. This interaction can be greater by introducing electrophilic and electron donative substituents into the .pi. electron conjugated system.
However, such organic compounds generally exhibit a great dipole moment which leads to a great interaction between dipoles upon crystallization, facilitating the formation of a crystal having a structure of symmetry about the center thereof in which the dipole moment of two molecules cancel each other. There is a problem that the secondary nonlinear optical effect, which is important in application, cannot be attained by such a crystal having a symmetry about the center thereof. In order to destroy the symmetry which prevents the attainment of light nonlinearity in crystal state, an approach has been made which comprises introducing substituents having a hydrogen bonding ability or optically active substituents containing asymmetric carbon atoms into an organic compound having .pi. electron conjugated system upon design of molecules. The inventors proposed a material comprising optically active substituents containing asymmetric carbon atoms incorporated in .pi. electron conjugated system in JP-A-3-112961 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, none of these proposed materials attain sufficient optical nonlinearity, storage stability and workability.
In general, materials for nonlinear optical element are required to exhibit enough light non-linearity, light transmission, laser resistance, crystallizability, phase adjustability, workability, mechanical strength, hygroscopicity, hardness, etc.
It has been extremely difficult to select among organic nonlinear optical element materials which have heretofore been known those satisfying the above mentioned practically necessary requirements.